1. Field of the Invention
The present invention relates in general to a fluid-filled resilient or elastic bushing structure, and more particularly to improvements in durability of the resilient or elastic member employed in the bushing and in vibration damping and isolating capability of the bushing.
2. Discussion of the Prior Art
There is known a resilient bushing for elastically connecting two members in a vibration system (through which vibrations are transmitted), for damping and/or isolating vibrations applied to the bushing in a given diametric direction of the bushing. The bushing has an inner sleeve in which a mounting rod or bolt is inserted, an outer sleeve on which a cylindrical mounting member is fitted, and a resilient member interposed between the inner and outer sleeves. For example, such a resilient bushing is used as a suspension bushing in a suspension system of an automotive vehicle, or an engine mount for mounting a power unit on the body of an F--F vehicle (front-engine, front drive vehicle).
Moreover, a fluid-filled resilient bushing has been proposed in recent years. An example of such a fluid-filled bushing is disclosed in U.S. Pat. No. 3,642,268. This bushing has a pair of fluid chambers formed in a resilient member such that the fluid chambers are located opposite to each other in a diametric direction of the bushing in which vibrations are applied. These fluid chambers are filled with a suitable incompressible fluid, and communicate with each other through an orifice, so that the fluid may flow through the orifice between the two chambers.
In the fluid-filled bushing indicated above, however, a portion of the resilient member is tensed when the bushing is mounted in place, i.e., placed under static load 1G, and consequently a comparatively large tensile strain is produced in the tensed portion of the resilient member. This disadvantage is common to another type of conventional fluid-filled bushing as disclosed in U.S. Pat. No. 3,698,703, in which the inner and outer sleeves are disposed in eccentric relation with each other such that the inner and outer sleeves are brought into concentric relation with each other when the bushing is mounted in place, for example, between a body of a vehicle and a power unit including an engine. To the "static" tensile strain produced in the tensed portion of the resilient member, is added a "dynamic" tensile strain due to a vibrational load externally applied to the bushing (resilient member). Thus, the tensed portion of the resilient member is subjected to the sum of the "static" and "dynamic" tensile strains when in use, and accordingly fissures tend to be produced in the resilient member.
In the conventional fluid-filled resilient bushings indicated above, the input low-frequency vibrations can be effectively damped due to inertia and resonance of the fluid mass in the orifice. However, if the orifice of this type of fluid-filled resilient bushing is dimensioned (in terms of its length and cross sectional area or diameter) so as to provide excellent damping characteristic for vibrations in a low frequency range, then the vibration isolating capability of the bushing is accordingly reduced for the high-frequency vibrations having a small amplitude. The conventional bushings having a pair of diametrically opposite fluid chambers cannot effectively isolate vibrations having a frequency higher than the resonance point of the fluid mass in the orifice, because the fluid mass in each fluid chamber becomes too rigid or stiff and accordingly the dynamic spring constant of the bushing becomes too high when such high-frequency vibrations are applied to the bushing.
The present inventor has proposed in Japanese patent application No. 59-267768, filed Dec. 19, 1984, a fluid-filled resilient bushing of a type which has a pressure-receiving chamber adapted to receive axial vibrations to be damped, and an equilibrium chamber partially defined by an elastically yieldable thin-walled partition member. The pressure-receiving chamber and the equilibrium chamber communicate with each other through an orifice, and elastic deformation of the partition member permits a change in the volume of the equilibrium chamber. In this arrangement, the volume of the pressure-receiving chamber can be changed with flows of the fluid between the two chambers through the orifice, accompanied by elastic deformation of the partition member of the equilibrium chamber. In this way, the bushing provides damping effect against vibrations axially applied thereto, however it is not adapted to damp vibrations radially applied thereto.